Whilst demons are no longer viewed as literal beings, as a metaphor the demon continues to trail ideas about doubt and truth, simulation and reality, into post- Enlightenment culture. This metaphor has been revitalised in a contemporary period that has seen the dominance of the cybernetic paradigm. Cybernetics has produced technologies of simulation, whilst the posthuman (a hybrid construction of the self emerging from cultural theory and technology) perceives the world as part of a circuit of other informational systems. In this thesis, illustrative films and literary fictions posit a connection between cybernetic epistemologies and metaphors of demonic possession, and contextualise these against postmodern thought and its narrative modes. Demons mark a return to pre-Enlightenment models of knowledge, so that demonic (dis)simulation can be seen to describe our encounters with artificial others and virtual worlds that reflect an uncertainly constituted and unstable self. By juxtaposing Renaissance notions of the demon with Donna Haraway's posthuman "cyborg," psychoanalytic demons with the robots of the science fiction film Forbidden Planet (1956), and Descartes' "deceiving demon" with Alan Turing's artificial intelligence test, I propose that the demon proves a fluid, multivalent trope that crosses historical and disciplinary boundaries. The demon raises epistemological questions about the relationship between reality, human psychology, and the representation of both in other modes, particularly narrative fictions. When this framework is applied to seminal science fiction (2001: A Space Odyssey and Do Androids Dream of Electric Sheep? [both 1968]), conventional readings of cyborgs as monstrous Others have to be revised. These fictions are engaged with cybernetic technologies with an epistemological rather than ontological concern, and consequently lend themselves to the kind of sceptical doubt about reality that characterises postmodern thought. Contrary to Descartes, who sees foundational truth through the deceptions of his "deceiving demon," later films like Blade Runner (1982) and The Matrix (1999) use the motif of cybernetic technologies to highlight the inescapability of the postmodern condition of the hyperreal. Finally, however, literary fictions like Umberto Eco's Foucault's Pendulum (1988) and A.S. Byatťs A Whistling Woman (2002) and Possession (1990) draw attention to their narrative mechanisms through metafiction, and set the creation of literary meaning against computer-generated texts. Consequently, they defy both the determinism of cybernetic sciences, and the postmodern pretence that the "real" is irrecoverably evasive.